They are the voracious Asian invaders that have evaded poisoning, lock gates and hi-tech electronic barriers as they penetrated thousands of miles of American rivers and canals, devouring most of the food in their path.

Now the silver and bighead carp, originally imported from Taiwan, face the might of the US Army in a last-ditch effort to prevent them from reaching the largest freshwater system in the world - the Great Lakes and their connecting rivers that straddle the Canadian border.

The carp, first brought to Arkansas in 1972 to help fish farms control water quality by consuming troublesome algae, have been relentlessly chomping their way north since the first handful escaped into the Mississippi River basin. They devour plankton in such quantity and with such efficiency, thanks to "gill rakers" attached to their heads, that native species struggle to compete and survive.

In November evidence was found that they were present in a canal seven miles from Lake Michigan, and now the fear is that the jumbo-sized aliens will endanger the ecosystem of the entire Great Lakes, squeeze out indiginous salmon, trout and perch by consuming their food sources and threaten the region's $7 billion fishing industry.

Illinois River silver carp
© AP
Illinois River silver carp jump out of the water after being disturbed by sounds of watercraft
They are also a serious hazard to the lives of boaters and water skiers because of their tendency to hurl themselves several feet out of the water - with an impact that has been compared to being struck by a bowling ball. The largest have grown to as much as 100lbs in weight.

"We brought these fish over to be environmental engineers so we should not be surprised how well they have done," said Duane Chapman, a fisheries biologist with the US Geological Survey. "They are the most efficient freshwater filter-feeding fish in the world."

From a Missouri research station that he calls "Carp Central", he is tracking the progress of the silver and bighead species north toward the Great Lakes, which cover thousands of square miles and contain more than one-fifth of the world's fresh surface water.

The prospect of their imminent arrival is viewed with understandable trepidation. "If we don't act now, their spread through the lakes is inevitable and likely to be catastrophic as they occupy some of the world's most productive fresh water," said Joel Brammeier, president of the Great Lakes Alliance.

In response, neighbouring states have petitioned the US Supreme Court, the highest tribunal in the land, to approve sending in the Army Corps of Engineers to seal off the locks on the Chicago Sanitary and Ship Canal - the sole water transport link between the Great Lakes and the Mississippi River basin.

Conservationists began to panic after DNA from Asian carp was found in the canal beyond an underwater electronic barrier whose power pulses were supposed to deter the carp from swimming any nearer to Lake Michigan.

That startling discovery fuelled existing fears that a flood could sweep the fish beyond the barrier, that the barrier might fail or that the carp could find their way into the lake through smaller waterways that run off the canal.

A previous attempt to block the canal temporarily to the carp by dumping poison in its water proved ineffective: thousands of native fish were killed but just one dead Asian carp was retrieved.

Illinois state, which administers the canal, objects to the plan to seal it off. Officials say that the threat is exaggerated, that the DNA evidence is inconclusive and that closure would damage the barge and shipping industries.

The attorney general of Michigan, backed by four other states that abut the lakes and the neighbouring Canadian province of Ontario, responded with their lawsuit - because Mr Brammeier believes that there is no time to lose. "We are not calling for a full closure of the canal, just a temporary shutdown of the locks while we work out what to do about this potentially devastating threat," he said.

"Whether it takes five or 10 years, these carp will spread through the lakes if they get in there, make no mistake. We have already seen the displacement of native fish by the carp in the rivers where they have spread.

"Our natural fish species are reliant on plankton, especially as juveniles, but the Asian carp strain the food supply out of the water with much greater efficiency. We have a $7 billion a year sports fishery industry that will be at risk because of this."

In the Mississippi, the bighead and silver carp have become dominant species, reaching weights of up to 100 lbs, while native fish have become notably skinnier as their food source diminishes.

Mr Brammeir also cited the safety dangers for boaters. "It's only good fortune that the these carp have not killed anyone ," he said. The silver carp are notorious for jumping up to six feet into the air when alarmed by the vibrations of motor engines in the water.

The spectacle may be thrilling to watch, but the carp have also regularly smashed into boats - and their occupants - with perilous consequences.

On the Mississippi river, one woman was knocked unconscious and overboard, saved from drowning only by her lifejacket, while a boy had his jaw broken and a man piloting a boat full of children temporarily lost control of the vessel when he was knocked over. All were victims of flying fish.

Mr Chapman recalled an occasion when passengers had to cling on desperately after a fish slammed into the throttle, sending the boat flying up a steep river bank.

"There's a lot of night-fishing for catfish on the Mississippi and sometimes you'll hear the sound of a carp leaving the water and you just duck down and put your head between your legs and hope for the best," said Mr Chapman.

"If you're heading down the river at 20 knots and one of these things strikes you, it's like being hit by a bowling ball. It's very dangerous, very painful and not at all rare."

The fish existed in low densities in the wild for about two decades, but then river monitors noted that their numbers increased geometrically in the early 1990s. They have since spread up the Mississippi as far as Iowa and down it to Louisiana as well as north-west along the Missouri river into South Dakota and north-east through the Illinois river.

To the ire of many in the Great Lakes area, the administration of President Barack Obama has sided with his home state of Illinois. Elena Kagan, his solicitor general, last week told the Supreme Court that the federal government did not believe the "dramatic step" of closing the canal locks was necessary.

On the campaign trail last year, Mr Obama staked his claim as the Great Lakes president and said he would have a "zero tolerance" policy for new invasive species. Many people in the Great Lakes believe that his administration is now selling them down the river.